Effects of Piptoporus betulinus Ethanolic Extract on the Proliferation and Viability of Melanoma Cells and Models of Their Cell Membranes

Piptoporus betulinus is a fungus known for its medicinal properties. It possesses antimicrobial, anti-inflammatory, and anti-cancer activity. In this study, several tests were performed to evaluate the cytotoxic effect of the ethanolic extract of Piptoporus betulinus on two melanoma human cell lines, WM115 primary and A375 metastatic cell lines, as well as Hs27 human skin fibroblasts. The extract proved to affect cancer cells in a dose-dependent manner, and at the same time showed a low cytotoxicity towards the normal cells. The total phenolic content (TPC) was determined spectrophotometrically by the Folin-Ciocalteu method (F-C), and the potential antioxidant activity was measured by ferric-reducing antioxidant power (FRAP) assay. One of the active compounds in the extract is betulin. It was isolated and then its cytotoxic activity was compared to the results obtained from the Piptoporus betulinus extract. To further understand the mechanism of action of the extract's anticancer activity, tests on model cell membranes were conducted. A model membrane of a melanoma cell was designed and consisted of 1,2-dimyristoyl-sn-glycero-3-phosphocholine, disialoganglioside-GD_1a and cholesterol: DMPC:GD_1a:chol (5:2:3 mole ratio). Changes in a Langmuir monolayer were observed and described based on Π-A_mol isotherm and compressibility modulus changes. LB lipid bilayers were deposited on a hydrophilic gold substrate and analyzed by IR and X-ray photoelectron spectroscopy. Our study provides new data on the effect of Piptoporus betulinus extract on melanoma cells and its impact on the model of melanoma plasma membranes.

Membrane Potentials Trigger Molecular-Scale Rearrangements in the Outer Membrane of Gram-Negative Bacteria

The structural complexity of the cell envelope of Gram-negative bacteria limits the fabrication of realistic models of bacterial cell membranes. A vertical Langmuir-Blodgett withdrawing was used to deposit a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) monolayer on the Au(111) surface. The second leaflet composed of di[3-deoxy-D-manno-octulosonyl]-lipid A (KLA) was deposited using Langmuir-Schaefer transfer. The use of an electrode material as a support for the POPE-KLA bilayer allowed electrochemical control of the membrane's stability, compactness, and structure. Capacitance-potential curves showed a typical pattern for the supported lipid bilayers electrochemical characteristic. The minimum membrane capacitance was ∼4 μF cm^-2 and did not change in the following desorption-adsorption cycles, indicating the presence of a stable bilayer structure with an asymmetric composition of both leaflets. However, at a molecular scale, as elucidated in spectroelectrochemical experiments, large differences in the response of both leaflets to electric potentials were observed. The acyl chains in POPE and KLA existed in a liquid state. The quantitative analysis of the CH stretching modes indicated potential-driven reorientations in the hydrophobic fragment of the bilayer, already in the adsorbed state. To assign observed rearrangements to POPE and KLA lipids in both leaflets, per-deuterated d₃₁-POPE was transferred into the inner leaflet. Since no potential-dependent changes of the CD₂ stretching modes in the d₃₁-POPE-KLA bilayer were observed, reorientations in the acyl chain region were assigned to the KLA molecules. Mg²⁺ ions were bound to the polar head groups of KLA. The strength of electrostatic interactions in the polar head group region of KLA was dependent on the direction of the electric field. At negative electric potentials, the binding of divalent cations weakened, which gave the KLA molecules increased orientational flexibility. This behavior in electric fields is peculiar for the outer membrane and indicates that the microbial cell membranes have different electrochemical properties than phospholipid bilayers.

The shape of lipid molecules affects potential-driven molecular-scale rearrangements in model cell membranes on electrodes

Planar asymmetric lipid bilayers composed of phosphatidylethanolamine and phosphatidylglycerol lipids are transferred onto a gold electrode surface. Lipids containing two saturated, one monounsaturated and two monounsaturated hydrocarbon chains compose the model membranes. Results of electrochemically controlled polarization modulation infrared reflection absorption spectroscopy and quartz crystal microbalance with energy dissipation studies reveal two different types of electric potential-dependent structural rearrangements in the bilayers. They are correlated with the geometry of the lipid molecule. Packing parameter correlates the cross-section area of the hydrophobic and hydrophilic parts of amphiphilic molecules. In bilayers composed of lipids with the packing parameter <1, the hydrocarbon chains are tilted with respect to the bilayer plane and the polar head groups are well hydrated. At a threshold potential an abrupt flow of water through the bilayer is connected with membrane dehydration and upward orientation of the chains. In bilayers composed of lipids with packing parameter ≥1, electric potentials have negligible effect on the membrane structure. A simple rule correlating the packing parameter with molecular scale changes occurring at electrified membranes has a large diagnostic implication for biomimetic studies and our understanding of molecular processes occurring in biological cell membranes.

Binding of a Myristoylated Protein to the Lipid Membrane Influenced by Interactions with the Polar Head Group Region

Many cytoplasmic proteins contain a hydrophobic acyl chain, which facilitates protein binding to cell membranes. Hydrophobic interactions between the exposed acyl chain of the protein and hydrocarbon chains of lipids in the cell membrane are the driving force for this specific lipid-protein interaction. Recent studies point out that in addition to hydrophobic interactions the charge-charge and charge-dipole interactions between the polar head groups and basic amino acids contribute significantly to the binding process. Recoverin possesses a myristoyl chain at the N-terminus. In the presence of Ca²⁺ ions, the protein undergoes structural rearrangements, leading to the extrusion of the myristoyl chain, facilitating the protein binding to the membrane. In this work, we investigate the impact of interactions between the polar head group region of lipid molecules and recoverin which binds to the model membrane. The interaction with a planar lipid bilayer composed of phosphatidylcholine and cholesterol with myristoylated and nonmyristoylated recoverin is studied by in situ polarization modulation infrared reflection absorption spectroscopy. The binding of recoverin to the lipid bilayer depends on the transmembrane potential, indicating that the orientation of the permanent surface dipole in the supramolecular assembly of the lipid membrane influences the protein attachment to the membrane surface. Analysis of the amide I' mode indicates that the orientation of recoverin bound to the lipid bilayer is independent of the presence of myristoyl chain in the protein and of the folding of the protein into the tense or relaxed state. In contrast, it changes as a function of the membrane potential. At positive transmembrane potentials, the α-helical fragments of recoverin are oriented predominantly parallel to the bilayer surface. This orientation facilitates the insertion of the acyl chain of the protein into the hydrophobic region of the bilayer. At negative transmembrane potentials, the α-helical fragments of recoverin change their orientation with respect to the membrane surface, which is followed by the removal of the myristoyl chain from the membrane.

PM-IRRAS Studies of DMPC Bilayers Supported on Au(111) Electrodes Modified with Hydrophilic Monolayers of Thioglucose

A phospholipid bilayer composed of 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (d54-DMPC) was deposited onto the Au(111) electrode modified with a self-assembled monolayer of 1-thio-β-d-glucose (β-Tg) via the Langmuir-Blodgett and Langmuir-Schaefer (LB-LS) techniques. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to characterize structural and orientational changes in this model biological membrane on a hydrophilic surface modified gold electrode. The results of the spectroscopic measurements showed that the tilt angle of acyl chains obtained for deuterated DMPC bilayers supported on the β-Tg-modified gold is significantly lower than that reported previously for DMPC bilayers deposited directly on Au(111) electrodes. Moreover, tilt angles of ∼18° were obtained for d54-DMPC bilayers on β-Tg self-assembled monolayers (SAMs) at positive potentials, which are similar to the values calculated for h-DMPC deposited on bare gold in the desorbed state and to those observed for a stack of hydrated DMPC bilayers. This data confirms that the β-thioglucose SAM promotes the formation of a water cushion that separates the phospholipid bilayer from the metal surface. As a result, the DMPC polar heads are not in direct contact with the electrode and can adopt a zigzag configuration, which strengthens the chain-chain interactions and allows for an overall decrease in the tilt of the acyl chains. These novel supported model membranes may be especially useful in studies pertaining to the incorporation of peptides and proteins into phospholipid bilayers.

Super-resolved FT-IR spectroscopy: Strategies, challenges, and opportunities for membrane biophysics

Direct correlation of molecular conformation with local structure is critical to studies of protein- and peptide-membrane interactions, particularly in the context of membrane-facilitated aggregation, and disruption or disordering. Infrared spectroscopy has long been a mainstay for determining molecular conformation, following folding dynamics, and characterizing reactions. While tremendous advances have been made in improving the spectral and temporal resolution of infrared spectroscopy, it has only been with the introduction of scanned-probe techniques that exploit the raster-scanning tip as either a source, scattering tool, or measurement probe that researchers have been able to obtain sub-diffraction limit IR spectra. This review will examine the history of correlated scanned-probe IR spectroscopies, from their inception to their use in studies of molecular aggregates, membrane domains, and cellular structures. The challenges and opportunities that these platforms present for examining dynamic phenomena will be discussed. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.

Electrochemical and PM-IRRAS characterization of cholera toxin binding at a model biological membrane

A mixed phospholipid-cholestrol bilayer, with cholera toxin B (CTB) units attached to the monosialotetrahexosylganglioside (GM1) binding sites in the distal leaflet, was deposited on a Au(111) electrode surface. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to characterize structural and orientational changes in this model biological membrane upon binding CTB and the application of the electrode potential. The data presented in this article show that binding cholera toxin to the membrane leads to an overall increase in the tilt angle of the fatty acid chains; however, the conformation of the bilayer remains relatively constant as indicated by the small decrease in the total number of gauche conformers of acyl tails. In addition, the bound toxin caused a significant decrease in the hydration of the ester group contained within the lipid bilayer. Furthermore, changes in the applied potential had a minimal effect on the overall structure of the membrane. In contrast, our results showed significant voltage-dependent changes in the average orientation of the protein α-helices that may correspond to the voltage-gated opening and closing of the central pore that resides within the B subunit of cholera toxin.

Biophysical investigation of retinal calcium sensor function

BACKGROUND

Neuronal calcium sensor proteins represent a subgroup of the family of EF-hand calcium binding proteins. Members of this subgroup are the guanylate cyclase-activating proteins and recoverin, which operate as important calcium sensors in retinal photoreceptor cells. Physiological and biochemical data indicate that these proteins participate in shaping the photoreceptor light response.

SCOPE OF REVIEW

Biophysical methods have been widely applied to investigate the molecular properties of retinal calcium binding proteins like the guanylate cyclase-activating proteins and recoverin. Properties include the determination of calcium affinities by isotope techniques and spectroscopical approaches. Conformational changes are investigated for example by tryptophan fluorescence emission. A special focus of this review is laid on a new experimental approach to study conformational changes in calcium binding proteins by surface plasmon resonance spectroscopy. In addition this technique has been employed for measuring the calcium-dependent binding of calcium sensors to membranes.

MAJOR CONCLUSIONS

Biophysical approaches provide valuable information about key properties of calcium sensor proteins involved in intracellular signalling. Parameters of their molecular properties like calcium binding and conformational changes help to define their physiological role derived from cellular, genetic or physiological studies.

GENERAL SIGNIFICANCE

Calcium is an important second messenger in intracellular signaling. Calcium signals are propagated via calcium binding proteins that are able to discriminate between incremental differences in intracellular calcium and that regulate their targets with high precision and specificity. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Electric field driven changes of a gramicidin containing lipid bilayer supported on a Au(111) surface

Langmuir-Blodgett and Langmuir-Schaeffer methods were employed to deposit a mixed bilayer consisting of 90% of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 10% of gramicidin (GD), a short 15 residue ion channel forming peptide, onto a Au(111) electrode surface. This architecture allowed us to investigate the effect of the electrostatic potential applied to the electrode on the orientation and conformation of DMPC molecules in the bilayer containing the ion channel. The charge density data were determined from chronocoulometry experiments. The electric field and the potential across the membrane were determined through the use of charge density curves. The magnitudes of potentials across the gold-supported biomimetic membrane were comparable to the transmembrane potential acting on a natural membrane. The information regarding the orientation and conformation of DMPC and GD molecules in the bilayer was obtained from photon polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) measurements. The results show that the bilayer is adsorbed, in direct contact with the metal surface, when the potential across the interface is more positive than -0.4 V and is lifted from the gold surface when the potential across the interface is more negative than -0.4 V. This change in the state of the bilayer has a significant impact on the orientation and conformation of the phospholipid and gramicidin molecules. The potential induced changes in the membrane containing peptide were compared to the changes in the structure of the pure DMPC bilayer determined in earlier studies.